This invention relates to a color picture tube apparatus, and more particularly a color picture tube apparatus provided with beam deflecting means which can automatically correct an image distortion and a convergence error.
FIG. 1 illustrates typical structure of a conventional color picture tube apparatus which comprises beam deflecting means and a color picture tube containing an electron gun of in-line type. The color picture tube is constituted by a rectangular face plate 1 with a phosphor screen 2 formed on its inner surface, a rectangular shadow mask 3 having a number of apertures through which electron beams pass, a funnel portion 4 connected to the face plate 1, and a neck portion 6 containing an electron gun 5 of in-line type. The phosphor screen 2 consists of a number of phosphor stripes which extend in vertical direction (or in FIG. 1, a direction perpendicular to the drawing sheet). When three electron beams 7 impinge upon each phosphor stripe, the stripe emits red, green and blue light, About the funnel portion 4 the beam deflecting means 8 is provided to deflect the electron beams 7 in horizontal and vertical directions so that an image is reproduced on the phosphor screen 2.
Generally, the beam deflecting means 8 not only deflects electron beams but also generates such a magnetic field as corrects a convergence error of three electron beams. The distributions of such a magnetic field are illustrated in FIG. 2A and FIG. 2B, wherein Z denotes the axis of the color picture tube, the intensity of a pincushion shaped magnetic field is plotted above axis Z, and that of a barrel shaped magnetic field of plotted below axis Z.
A horizontal magnetic field has such an intensity distribution that, as curve 9 in FIG. 2A shows, a barrel shaped magnetic field is located near the electron gun 5 and a pincushion shaped magnetic field near the phospher screen 2. On the other hand, a vertical magnetic field exhibits such an intensity distribution that, as curve 10 in FIG. 2B show, a barrel shaped magnetic field extends from the electron gun 5 to the phosphor screen 2.
With the beam deflecting means 8 generating such a magnetic field distribution it is possible to automatically correct a convergence error at the central part and edge parts of the phosphor screen 2. However, a raster formed on the phosphor screen 2 is inevitably distorted as indicated by dotted line 12 in FIG. 3. Ideally, every raster should be so rectangular as indicated by solid line 11 in FIG. 3. Usually such a raster distortion is at most about 1% in Y axis. The distortion in X axis, however, amounts to about 3% to 5% in a color picture tube of 90!-deflection and about 7% to 8% in a color picture tube of 110!-deflection. The raster distortion is acutest at the four corners 13 of the phosphor screen 2.
Various method have been invented to reduce the above-mentioned raster distortion.
The first method uses such a distortion correction circuit as illustrated in FIG. 4A. A portion of current flowing from a vertical deflection output (V output) to a vertical deflection coil (V coil) is superposed by means of a transformer T on the current which flows from a horizontal deflection output (H output) to a horizontal coil (H coil), whereby a horizontal deflection current is modulated in synchronism with the vertical deflection so as to change a distorted raster shown in FIG. 4B to a correctly shaped raster indicated by dotted line 15 in FIG. 4B. The horizontal deflection current thus modulated has such a waveform as illustrated in FIG. 4C. The use of the correction circuit of FIG. 4A, however, renders the circuit of the color television receiver complicated and increases the cost of the color television receiver.
The second method uses such a toroidal type vertical deflecting means 18 as shown in FIG. 5, which generates a magnetic field of such an intensity distribution as would simulteneously correct both a convergence error and a raster distortion. To generate such a magnetic field, the toroidal type vertical deflection coil of the means 18 must be wound at an angle on the electron gun side and at another angle on the phosphor screen side. For this purpose, the core 17 of the means 18 should have a bend portion 19 as shown in FIG. 5. However, it is difficult to wind a wire about a core having such a bent portion to form a vertical deflection coil which can generate a magnetic field of such an intensity distribution as would correct both a convergence error and a raster distortion. Beam deflecting means, if manufactured in large quantities, would have different characteristics and would therefore reproduce images of different qualities.
The third method uses a plurality of permanent magents within or in the vicinity of a beam deflecting means. The magnets generates a static magnetic field to correct a convergence error and a raster distortion. This magnetic field, however, acts locally on a raster. It is static and it is increasingly more intense toward the magents. The static magnetic field corrects such a distorted raster as shown in FIG. 4B but locally as indicated by dotted line 21 in FIG. 6. The raster thus corrected is, therefore, still distorted locally, and distortion is created at the central part of the phosphor screen 2 as illustrated in FIG. 7. Such distortion will deteriorate the quality of reproduced images.